1. Field of the Invention
The present invention relates to dynamoelectric machines such as large induction motors, and more particularly this invention relates to an induction motor for driving large inertia loads having a hollow ferromagnetic rotor and means for transferring heat therefrom.
2. Description of the Prior Art
Various methods are available for starting reversible pumped storage generator-motors. One method currently used is by means of a starting motor which is mounted directly on the main machine shaft. In such an arrangement a large wound rotor motor is often used. This motor is rather expensive and requires a rheostat (usually a water rheostat) to dissipate the energy involved during starting. This energy is, as a minimum, equal to H.KVA of the large machine being started where H is the inertia constant in KW seconds per KVA stored energy of rotation. This is further increased by the speed difference of the wound rotor induction motor synchronous speed compared to the rated speed of the large synchronous motor. It is also increased by the torque required to overcome friction and windage losses in both the generator motor and pump compared to the torque which is directly applied to the inertia for acceleration.
The rotor assemblies for starting motors suitable for use with hydrogenerators are relatively large in physical size and, although they rotate at relatively low speeds., their large diameter and great weight result in moderately high centrifugal forces at the periphery of the rotor during operation. The rotational forces in combination with the differential heating caused by the circulation of heavy induced currents during starting make the ordinary squirrel cage rotor assembly unsuitable for this application since the rotor bars tend to heat and expand unevenly and cannot withstand the large mechanical and thermal stresses.
The mechanical and thermal problems involved in this type construction have been increasingly severe, and alternative starting arrangements such as asynchronous starting at full or reduced line voltage have been utilized. However, the disturbing effects of large current inrush on the interconnected network and heating of the damping windings are sometimes encountered when the asynchronous starting method at full or reduced line voltage is used, so that satisfactory designs of the this type are not always possible.